Attach is a technical state that various card types can acquire. It represents the card being under the ownership, placement, or otherwise assimilated by the attached target.
Various things in the game can be the target of attachable cards, such as Creatures, Items, Zones, and even Players.
Attach versus Equip
Equip is an action a player can use to attach Item and Skill cards to targets.
Since Items and Skills enter play unattached to anything, the player may wait and decide later how to use them. At that point, they pay the Equip cost and attach the card to its target.